I. Field of the Invention
This invention relates to valve control systems for selectively operating an internal combustion engine in either a power mode or a retarding mode.
II. Discussion of the Prior Art
While the advantages of obtaining a braking effect from the engine of a vehicle powered by an internal combustion engine are well known (see for example U.S. Pat. No. 3,220,392 to Cummins) an ideal braking system design characterized by low cost, simplicity, ease of maintenance and reliability has not yet been achieved. One well known approach has been to convert the engine into a compressor by cutting off fuel flow and, opening the exhaust valve for each cylinder near the end of the compression stroke and to close the exhaust valve shortly thereafter; thus, permitting the conversion of the kinetic inertial energy of the vehicle into compressed gas energy which may be released to atmosphere when the exhaust valves are partially opened. To operate the engine reliably as a compressor, rather exacting control is necessary over the timed relationship of exhaust valve opening and closing relative to the movement of the associated piston. One technique for accomplishing this result is disclosed in U.S. Pat. No. 3,786,792 to Pelizzoni et al, wherein the exhaust valve train of an engine is provided with a dual ramp cam and cooperating, hydraulically operated tappet to selectively open and close the exhaust valve as necessary to operate the engine as a gas compressor. The engine braking system of Pelizzoni et al, is desirable and useful in many engine environments but does present cost obstacles when it is desired to retrofit an existing engine since special dual ramp cams must be substituted for the standard exhaust valve cams normally provided in an engine.
An alternative and somewhat less expensive hydraulic system may be employed in certain internal combustion engines by the provision of a slave hydraulic piston for opening an exhaust valve near the end of the compression stroke of an engine piston with which the exhaust valve is associated. The slave piston which opens the exhaust valve is actuated by a master piston hydraulically linked to the slave piston and mechanically actuated by an engine element which is displaced periodically in timed relationship with the compression stroke of the engine piston. One such engine element may be the intake valve train of another cylinder timed to open shortly before the first engine cylinder piston reaches the top dead center of its compression stroke. Other engine operating elements may be used to actuate the master piston of the braking system so long as the actuation of the master piston occurs at the proper moment near the end of the compression stroke of the piston whose associated exhaust valve is to be actuated by the slave piston. For example, certain types of compression ignition engines are equipped with fuel injector actuating mechanisms which are mechanically actuated near the end of the compression stroke of the engine piston with which the fuel injector valve train is associated thus providing an actuating mechanism immediately adjacent the valve which is to be opened all as illustrated in the Cummins U.S. Pat. No. 3,220,392 patent and as further described in U.S. Pat. No. 3,405,699 to Laas.
The use of hydraulically linked master/slave pistons in a system for selectively converting an internal combustion engine from a power mode to a compressor mode of operation has proven to be commercially viable and to be relatively simple especially in engines already equipped with appropriately timed fuel injector actuating mechanisms. However, certain difficulties have arisen in controlling the amount of energy required to operate the slave piston. These difficulties appear to result from variations in engine timing, turbo charging and compression ratio. In particular, it has been found that in the operation of the above described system, frequently, the fuel injector valve trains deform due to undesirably excessive loading by the master pistons, resulting in costly repairs and a total loss in vehicle usage for a lengthy period of time. Prior to the subject invention, no technique had been employed to control effectively and efficiently variations in the energy required by a slave/master hydraulic braking system used to operate an internal combustion engine in a braking mode as a compressor.